If you're willing to hack your empeg, trace the microphone wire through the docking connector, and snip it carefully on the internal harness. I'd advise not snipping too close to either connector so that you could resplice it later if need be. Patch the wire to an appropriate internal connection. Someone has already suggested a GPIO. Carefully remove the microphone wire from the harness and replace the appropriate pin with one wired for your use. All at your own risk of course.

The player's serial port is DCE so that it can be connected with a null modem cable to a PC (which are usually DTE). The sled connector on the other hand is a modified DTE with only TX and RX brought out. Pin 4 (usually DCE Ready) has the amp remote line on it. This is 12V switched with the VFD screen - ie, if the VFD is on this is 12V. You cannot control this separately.